Inline power (also sometimes referred to as Power over Ethernet and PoE) is a technology for providing electrical power over a wired telecommunications network from power source equipment (PSE) to a powered device (PD) over a link section. The power may be injected by an endpoint PSE at one end of the link section or by a midspan PSE along a midspan of a link section that is distinctly separate from and between the medium dependent interfaces (MDIs) to which the ends of the link section are electrically and physically coupled.
A form of PoE is defined in the IEEE (The Institute of Electrical and Electronics Engineers, Inc.) Standard Std 802.3af-2003 published 18 Jun. 2003 and entitled “IEEE Standard for Information technology—Telecommunications and information exchange between systems—Local and metropolitan area networks—Specific requirements: Part 3 Carrier Sense Multiple Access with Collision Detection (CSMA/CD) Access Method and Physical Layer Specifications: Amendment: Data Terminal Equipment (DTE) Power via Media Dependent Interface (MDI)” (herein referred to as the “IEEE 802.3af standard”). The IEEE 820.3af standard is a globally applicable standard for combining the transmission of Ethernet packets with the transmission of DC-based power over the same set of wires in a single Ethernet cable. It is contemplated that Inline power will power such PDs as Internet Protocol (IP) telephones, surveillance cameras, switching and hub equipment for the telecommunications network, biomedical sensor equipment used for identification purposes, other biomedical equipment, radio frequency identification (RFID) card and tag readers, security card readers, various types of sensors and data acquisition equipment, fire and life-safety equipment in buildings, and the like. The power is direct current, 48 Volt power available at a range of power levels from roughly 0.5 watt to about 15.4 watts in accordance with the standard. There are mechanisms within the IEEE 802.3af standard to allocate a requested amount of power. Other proprietary schemes also exist to provide a finer and more sophisticated allocation of power than that provided by the IEEE 802.3af standard while still providing basic compliance with the standard. As the standard evolves, additional power may also become available. Conventional 8-conductor type RG-45 connectors (male or female, as appropriate) are typically used on both ends of all Ethernet connections. They are wired as defined in the IEEE 802.3af standard. Two conductor wiring such as shielded or unshielded twisted pair wiring (or coaxial cable or other conventional network cabling) may be used so each transmitter and receiver has a pair of conductors associated with it.
FIGS. 1A, 1B and 1C are electrical schematic diagrams of three different variants of PoE as contemplated by the IEEE 802.3af standard. In FIG. 1A a data telecommunications network 10a comprises a switch or hub 12a with integral power sourcing equipment (PSE) 14a. Power from the PSE 14a is injected on the two data carrying Ethernet twisted pairs 16aa and 16ab via center-tapped transformers 18aa and 18ab. Non-data carrying Ethernet twisted pairs 16ac and 16ad are unused in this variant. The power from data carrying Ethernet twisted pairs 16aa and 16ab is conducted from center-tapped transformers 20aa and 20ab to powered device (PD) 22a for use thereby as shown. In FIG. 1B a data telecommunications network 10b comprises a switch or hub 12b with integral power sourcing equipment (PSE) 14b. Power from the PSE 14b is injected on the two non-data carrying Ethernet twisted pairs 16bc and 16bd. Data carrying Ethernet twisted pairs 16ba and 16bb are unused in this variant for power transfer. The power from non-data carrying Ethernet twisted pairs 16bc and 16bd is conducted to powered device (PD) 22b for use thereby as shown. In FIG. 1C a data telecommunications network 10c comprises a switch or hub 12c without integral power sourcing equipment (PSE). Midspan power insertion equipment 24 simply passes the data signals on the two data carrying Ethernet twisted pairs 16ca-1 and 16cb-1 to corresponding data carrying Ethernet twisted pairs 16ca-2 and 16cb-2. Power from the PSE 14c located in the Midspan power insertion equipment 24 is injected on the two non-data carrying Ethernet twisted pairs 16cc-2 and 16cd-2 as shown. The power from non-data carrying Ethernet twisted pairs 16cc-2 and 16cd-2 is conducted to powered device (PD) 22c for use thereby as shown. Note that powered end stations 26a, 26b and 26c are all the same so that they can achieve compatibility with each of the previously described variants.
Turning now to FIGS. 1D and 1E, electrical schematic diagrams illustrate variants of the IEEE 802.3af standard in which 1000 Base T communication is enabled over a four pair Ethernet cable. Inline power may be supplied over two pair or four pair. In FIG. 1D the PD accepts power from a pair of diode bridge circuits such as full wave diode bridge rectifier type circuits well known to those of ordinary skill in the art. Power may come from either one or both of the diode bridge circuits, depending upon whether inline power is delivered over Pair 1-2, Pair 3-4 or Pair 1-2 +Pair 3-4. In the circuit shown in FIG. 1E a PD associated with Pair 1-2 is powered by inline power over Pair 1-2 and a PD associated with Pair 3-4 is similarly powered. The approach used will depend upon the PD to be powered. In accordance with both of these versions, bidirectional full duplex communication may be carried out over each data pair, if desired.
Inline power is also available through techniques that are non-IEEE 802.3 standard compliant as is well known to those of ordinary skill in the art.
In order to provide regular inline power to a PD from a PSE it is a general requirement that two processes first be accomplished. First, a “discovery” process must be accomplished to verify that the candidate PD is, in fact, adapted to receive inline power. Second, a “classification” process must be accomplished to determine an amount of inline power to allocate to the PD, the PSE having a finite amount of inline power resources available for allocation to coupled PDs.
The discovery process looks for an “identity network” at the PD. The identity network is one or more electrical components which respond in certain predetermined ways when probed by a signal from the PSE. One of the simplest identity networks is a resistor coupled across the two pairs of common mode power/data conductors. In accordance with the IEEE 802.3af standard, a 25,000 ohm resistor, for example, may be presented for discovery by the PD. The resistor may be present at all times or it may be switched into the circuit during the discovery process in response to discovery signals from the PSE.
The PSE applies some inline power (not “regular” inline power, i.e., reduced voltage and limited current) as the discovery signal to measure resistance across the two pairs of conductors to determine if the identity network is present. This is typically implemented as a first voltage for a first period of time and a second voltage for a second period of time, both voltages exceeding a maximum idle voltage (0-5 VDC in accordance with the IEEE 802.3af standard) which may be present on the pair of conductors during an “idle” time while regular inline power is not provided. The discovery signals do not enter a classification voltage range (typically about 15-20V in accordance with the IEEE 802.3af standard) but have a voltage between that range and the idle voltage range. The return currents responsive to application of the discovery signals are measured and a resistance across the two pairs of conductors is calculated. If that resistance is the identity network resistance, then the classification process may commence, otherwise the system returns to an idle condition.
In accordance with the IEEE 802.3af standard, the classification process involves applying a voltage in a classification range to the PD. The PD may use a current source to send a predetermined classification current signal back to the PSE. This classification current signal corresponds to the “class” of the PD. In the IEEE 802.3af standard as presently constituted, the classes are as set forth in Table I:
TABLE IPSE ClassificationCorrespondingClassCurrent Range (mA)Inline Power Level (W)00-515.41 8-134.0216-217.0325-3115.4435-45Reserved
The discovery process is therefore used in order to avoid providing inline power (at full voltage of −48 VDC) to so-called “legacy” devices which are not particularly adapted to receive or utilize inline power.
The classification process is therefore used in order to manage inline power resources so that available power resources can be efficiently allocated and utilized. Accordingly, network devices (such as Ethernet switches, for example) acting as PSEs to provide inline power to PDs keep track of the class of the attached PD (and therefore its worst case power draw) by maintaining a database in memory at the network device. This function is normally managed by the network device operating system (referred to as “IOS” or the “Internetworking Operating System” in Cisco Systems, Inc.'s network switches).
Uninterruptible Power Supply (UPS) backup is an expensive but important resource in data telecommunications networks. Inline power applications obviously require additional power resources beyond those necessary to simply power the network devices themselves and assuring continued operation of such applications in the event of a power failure requires substantially larger UPS resources than previously required.
FIG. 2 is a block diagram illustrating a data telecommunications network configuration having a single network device (switch) and a UPS device providing power backup resources to a single power supply associated with the network device in accordance with the prior art.
In the configuration shown in FIG. 2 UPS 30 obtains its power from AC mains supply 32. UPS 30 may be any appropriate form of high-reliability power supply such as a conventional battery-powered inverter, generator, or the like. The UPS 30 provides reliable power to a power supply 34 of network device 36 over line 37. Network device 36 is configured as a PSE device with PSE ports 38a, 38b, 38c and 38d. Corresponding PDs 40a, 40b, 40c and 40d receive inline power from these corresponding ports.
FIG. 3 is a block diagram illustrating a data telecommunications network configuration having a single network device (switch) and a UPS device providing power backup resources to a pair of power supplies associated with the network device in accordance with the prior art.
The configuration shown in FIG. 3 differs slightly from that of FIG. 2 in that network device 36′ has two power supplies 34a and 34b which are powered over corresponding lines 37a and 37b. In a configuration like that'shown, the supplies 34a, 34b may be fully redundant supplies or one may be configured to supply power to the network device circuitry while the other is dedicated to supplying power to the PSE circuitry for powering the attached PDs.
As the number of inline power applications and the power demand per inline power application increases in conjunction with increased critical and high availability requirements for inline power, there is an increased demand on UPS systems. Nevertheless, UPS systems continue to be a relatively high-cost and limited resource. UPS system costs, rack space, thermal dissipation, battery requirements and power outlet requirements are some of the constraints faced by those intending to deploy such systems. It is therefore not uncommon to implement a system where a single UPS system is shared or pooled across a number of network devices (such as Ethernet switches).
FIG. 4 is a block diagram illustrating a data telecommunications network configuration having a pair of network devices (switches) and a UPS device providing power backup resources to individual power supplies associated with the pair of network devices in accordance with the prior art.
In the configuration illustrated in FIG. 4 UPS 30 provides UPS resources to both network device 36-1 (via power supply 34-1 and line 37-1) and network device 36-2 (via power supply 34-2 and line 37-2). Network device 36-1 has PSE ports 38-1a through 38-1d and powers corresponding PDs 40-1a through 40-1d. Network device 36-2 has PSE ports 38-2a through 38-2d and powers corresponding PDs 40-2a through 40-2d. 
When more than one PSE device such as inline power enabled Ethernet switches are coupled to a single UPS resource, at least two problems arise. First, the UPS resource may become inadvertently oversubscribed beyond its current- or power-delivering capability in providing backup power for the plural network devices. Moreover, if such a scenario exists, then it may only become apparent for the first time when the power fails resulting in an unpredictable and possibly undesirable outcome. Second, those deploying such systems are faced with the difficult decision of whether to spend money up front by planning for the worst case load based on the maximum potential inline power load even though the system may always operate at a fraction of that load (e.g., due to the fact that PDs are not coupled to every port), or to try to schedule additional UPS resource purchases to match inline power utilization growth. As a result the customer may either be forced to buy expensive resources that may never be used or suffer service interruptions during power disruptions due to deploying inadequate resources.
Accordingly, it would be desirable to allow customers to place a number of inline power-providing network devices on the same UPS resource without having to worry about supplying resources for the worst case scenario up front. Moreover, it would be desirable to allow the customers to over-subscribe the use of the UPS resource among a plurality of network devices while having the comfort of an early indication or warning when the aggregate load across all of the network devices enters a range close to but not exceeding the existing UPS resource capability.